1. Field of the Invention
This invention relates to a radiation image storage panel, and more particularly relates to a radiation image storage panel comprising a support, a phosphor layer provided thereon which comprises a binder and stimulable phosphor particles dispersed therein, and a protective film provided on the phosphor layer.
2. Description of Prior Arts
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a silver salt sensitive material and an intensifying screen.
As a method replacing the above-described radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, is recently paid much attention. In the radiation image recording and reproducing method, a radiation image storage panel comprising a stimulable phosphor (a stimulable phosphor sheet) is used, and the method involves steps of causing the stimulable phosphor of the panel to absorb a radiation energy having passed through an object or having radiated by an object; exciting the stimulable phosphor with an electromagnetic wave such as visible light and infrared rays (hereinafter referred to as "stimulating rays") to sequentially release the radiation energy stored in the stimulable phosphor as light emission; photo-electrically processing the emitted light to give an electric signal; and reproducing the electric signal as a visible image on a recording material such as a radiographic film or on a recording apparatus such as CRT.
In the above-described radiation image recording and reproducing method, a radiation image can be obtained with a sufficient amount of information by applying a radiation to the object at considerably smaller dose, as compared with the case of using the conventional radiography. Accordingly, this radiation image recording and reproducing method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the above-described radiation image recording and reproducing method has a basic structure comprising a support and a phosphor layer provided on one surface of the support. Further, a transparent film is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. When excited with stimulating rays after having been exposed to a radiation such as X-rays, the stimulable phosphor particles emit light (stimulated emission). Accordingly, the radiation having passed through an object or having radiated by an object is absorbed by the phosphor layer of the radiation image storage panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the radiation image storage panel in the form of a radiation energy-stored imagea (a latent image). The radiation energy-stored image can be released as stimulated emission (light emission) by applying stimulating rays to the panel. The stimulated emission is photo-electrically processed to convert to electric signals, so as to produce a visible image from the radiation energy-stored image.
It is desired for the radiation image storage panel employed in the radiation image recording and reproducing method to have a high sensitivity and to provide an image of high quality (shapness, graininess, etd.) As described hereinbefore, the radiation image storage panel generally has a protective film to keep the phosphor layer from chemical deterioration or physical shock. As the protective film provided on the phosphor layer, a film having very high optical transparency is proposed in order to obtain an image of high quality without lowering of the image sharpness. Examples of such highly transparent protective film materials include a variety of plastic films available in the market which have a haze value [defined in JIS (Japanese Industrial Standard) K 6714] within the range of 2-3%.
In a radiation image obtained upon exciting the radiation image storage panel with stimulating rays after exposure to a radiation such as X-rays, certain shaded portions are sometimes observed in addition to the desired radiation image of the object, resulting in production of an image having unevenness in optical density. In an extreme case, certain visible line patterns are produced in the resulting image. As a light source of stimulating rays, a laser beam showing a high beam convergence is generally employed, and in the case of using the laser beam, the unevenness in optical density is particularly frequently observed. The unevenness in optical density of image causes troubles in analysis of the image, which resulting in lowering of quality and amount of information on the object.